A crystal chemistry study of LiNi1-yCoyO2 phases, used as positive electrode in lithium batteries, is presented. These materials crystallize in the rhombohedral system (space group : ) with a layered structure. Rietveld profile refinement of the X-ray data shows that for low substitution amounts (y less than or equal to 0.20) extra-nickel ions are always present leading to the Li1-zNi1+z-1Co1O2 (t=y(1+z)) formula (z>0), while for y greater than or equal to 0.30, a pure 2D structure is obtained (z=0). The stabilization of the 2D character of the structure by cobalt substitution in lithium nickelate leads to the improvement of the electrochemical properties.